This invention relates to a demagnetizing method for a thermo-magnetic recording operation.
In magnetic recording methods, a magnetic latent image is formed in a magnetic material by magnetization and is then made visible by magnetic toner particles, namely, magnetization detection type coloring particles which include magnetic particles in macromolecular resin, for instance, and which are affected by a magnetic field. The visible image thus obtained is transferred onto a sheet or the like by an electrostatic or magnetic method, and is then fixed by heat or pressure.
The same process is again carried out when the magnetic latent image carrier, namely, the magnetic recording medium, after being subjected to magnetic toner removal, is directly moved to the next developing cycle, or when, with the magnetic latent image erased, a new latent image has been formed.
A variety of methods of forming magnetic latent images have been proposed for the above-described magnetic recording method. Among these methods, the so-called thermo-magnetic recording method in which a magnetic latent image is formed according to a thermal input is excellent in that an inexpensive heating head array can be used to form the latent image.
With respect to latent image formation methods for thermo-magnetic recording and the thermo-magnetic recording medium used therein, the present applicant has proposed a magnetic recording method which is excellent in image quality when the latent image is visualized with toner particles, without using an external bias magnetic field, using a thermo-magnetic recording method, especially a thermal residual magnetization method (cf. Japanese Patent Application No. 37865/1981).
This invention provides a magnetic latent image erasing method which can be satisfactorily employed with the thermo-magnetic recording method disclosed by the aforementioned Japanese patent application.
As conducive to a full understanding of this invention, the thermo-magnetic recording method proposed by the application will be briefly described.
FIG. 1 is an explanatory diagram for describing the recording medium which is employed in the thermo-magnetic recording method.
The recording medium 1, as shown in FIG. 1(a), is made up of a non-magnetic base layer 1, a first magnetic layer 2 which is magnetized in advance, thus having a magnetization pattern 5, a non-magnetic intermediate layer 3, and a second magnetic layer 4.
The non-magnetic intermediate layer 3 may be eliminated as the case may be. A protective layer (not shown) may be formed on the surface of the second magnetic layer.
FIG. 1(a) shows the magnetic state of the recording medium before the input of the thermal pattern. That is, a repetitive magnetization pattern as indicated at 5 is provided over the entire first magnetic layer 2.
In this case, it can be considered that the magnetic flux 6 which is produced from the magnetization pattern 5 is distributed in the intermediate layer 3 and the second magnetic layer 4. However, the magnetic field H due to the magnetic flux which acts on the second magnetic layer is selected so as to be smaller than the coercive field HC(To) at a temperature To which is, for instance, the temperature of the environmental atmosphere when no thermal pattern is applied to the second magnetic layer.
Accordingly, in the case of FIG. 1(a) the magnetic recording medium contains no image data.
A thermal pattern is then formed on the second magnetic layer 4 of the magnetic recording medium by selectively exposing the latter to light, by allowing the same to contact a thermal head, or by applying a laser beam spot to the same. In this case, in the medium, the high temperature state is represented by a temperature T.sub.2, and the low temperature state is represented by a temperature T.sub.1, such that, for instance (T.sub.0 .ltoreq.T.sub.1 &lt;T.sub.2).
If, in this case, the second magnetic layer is made of a material whose coercive field Hc changes with temperatures as indicated in FIG. 2(a), only the high temperature portion T.sub.2 is selectively subjected to residual magnetization.
FIG. 2(a) is a graphical representation indicating the temperature dependence of the coercive field, Hc, which is one of the thermo-magnetic efects. In FIG. 2(a) reference Tc designates the Curie temperature.
The thermal residual magnetization phenomenon in the invention can be described clearly with reference to FIG. 2(b). Thermal residual magnetization is a phenomenon in which, when a magnetic material is cooled down to room temperature (=T.sub.0 .perspectiveto.T.sub.1) after being heated to an initial temperature T.sub.2 under the application of an external magnetic field H (the horizontal axis of FIG. 2(b)), the magnetic material has a thermal residual magnetization of Mr(T.sub.2).
Accordingly, the process from FIGS. 1(b) to 1(c) can be described with reference to FIG. 2(b) as follows:
The magnetic field H (as indicated in FIG. 2(b)) produced by the first magnetic layer acts on the second magnetic layer 4. As the high temperature portion T.sub.2 only is cooled to T.sub.0 from high temperatures T.sub.2 ' or T.sub.2 ", it exhibits thermal residual magnetization of Mr(T.sub.2), Mr(T.sub.2 ') or Mr(T.sub.2 "), while the low temperature portion T.sub.1 scarcely exhibits residual magnetization.
Through the above-described operation, a magnetic latent image is formed in correspondence to the thermal pattern without applying a magnetic field from an external source.
The second magnetic layer is made of a magnetic material which exhibits the above-described thermal residual magnetization phenomenon. In this connection, it is preferable to select a magnetic material whose thermal residual magnetization occurs in a high temperature range which is relatively close to room temperature. It is more preferable to use as the second magnetic layer a dispersion coat type CrO.sub.2 (chromium dioxide) magnetic layer whose Curie temperature is about 130.degree. C., or a non-crystalline film of a rare earth metal--transition metal alloy (Tb-Fe, or Gd-Fe).
The magnetic latent image which has been formed thermo-magnetically as described above is made visible by a magnetism detecting fine powder known commercially as one-component magnetic toner powder.
After suitable treatment including a magnetic latent image visualizing step, the magnetic latent image is erased, so that the thermo-magnetic recording operation may be repeated.
In erasing the magnetic latent image, it is necessary that the magnetization pattern over the entire first magnetic layer remain and only the magnetization pattern formed on the second magnetic layer be selectively erased.
In this case, one may consider utilizing the fact that the magnetic field from a magnetic head decreased abruptly away from the head surface, as disclosed by the above noted application; i.e., one may consider using an erasing magnetic head which is energized by a high frequency power source and has a long track width.
In the above-described AC magnetic field application method in which a magnetic head having a long track width is used, the head must be considerably high in uniformity in the longitudinal direction in order that it may be brought in satisfactorily close contact with the recording medium, with a result that the head becomes high in manufacturing cost.